The air intake ducts for a modern jet aircraft are structurally preceded with a diffuser that reduces air velocity entering a jet engine compressor intake so that the engine thrust is maximized. For military aircraft it is well known that, for forward sector engagement, the inlet for the aircraft's propulsion system contributes to a high radar cross section signature which is easily detectable. In addition, fore and aft visible "hot spots" produced by propulsive machinery can provide bright targets for heat-seeking missiles. Thus, there is a need for drastic reduction of infrared emission and radar cross section for the aircraft inlet and engine nozzle.
One of the instinctive methods to minimize the infrared emissions and microwave reflections of aircraft propulsive inlets is to introduce considerable offset between the centerlines of the entry and exit, thereby creating an elongated S-shaped diffuser. Other methods are to design the air inlet as narrow, high aspect ratio lenticular slits or apertures that conform to the external structural surfaces of the aircraft.
However, these techniques may lead to nonuniform flow profiles (i.e., flow "quality") and inadequate ram air pressure recovery at the compressor inlet station. The air supply needs of the engine throughout the flight envelope pose consideration of innovative fluid dynamical solutions for both aspects of the overall problems, i.e., reduced observables and adequate propulsion performance to perform and survive a military mission.